The present invention relates to an apparatus and a method of measuring the pressure distribution generated by a three-dimensional object, in particular the feet of humans or animals. The present invention is particularly useful in the diagnosis of leg and foot abnormalities and in the selection and design of foot wear.
Several devices have been designed to measure forces applied by the foot. These include devices for measuring applied forces that are attached to the foot during walking or running, for measuring peak pressure, for measuring the total force applied by the foot, and for measuring forces applied by certain areas of the foot for various purposes. Other devices employ a stationary foot sensor pad and are suited for use in a laboratory or hospital.
Conventional foot force measurement devices function to convert mechanical force into a suitable electrical signal. The types of sensors used include pneumatic or hydraulic fluid activated switches, strain gauge sensors that respond to mechanical deformation, single direct electronic force sensors or multiple direct electronic force sensors.
Pneumatic and hydraulic sensors are prone to produce inaccurate readings because changes in internal pressure are not necessarily proportional to change in forces. One way to construct an accurate fluid sensor is to use rigid plates which limit the applied force to a constant area of fluid contact. In such sensors, the fluid pressure is proportional to the applied force. However, these sensors are disadvantageous due their bulkiness and weight.
The sensors which measure mechanical deformation of structural elements supporting the wearer""s foot by use of electrical wire or ribbon type strain gauges accurately measure weight, but they are also disadvantageous because of their bulk and weight.
In direct electronic force sensor devices, the applied pressure compresses a partially conductive material (carbon granules) and thereby reduces its electrical resistance. A force sensor based on this concept can be constructed by forming thin electrodes on two plastic films and then covering the electrodes with a thin layer of suitable partially conductive material. When the two coated films are positioned so that the two layers of partially conductive material are in contact, the electrical resistance through the sensor from electrode to electrode varies inversely with a compressing force on the sensor. This is because as the coating is pressed with greater force, the electrical current path becomes broader due to increased area of contact. This effect occurs with any conductive material, but materials useful as active elements in practical force sensors must have a relatively high electrical resistance. Such thin film electronic force sensors have the advantages of simplicity, compactness, and light weight, but the disadvantage that they only provide relative indications of pressure applied.
In multiple direct electronic force sensors the sensors may be regularly spaced. Since the relative position of each sensor is fixed, a mathematical description of the location of each sensor can easily be made part of a permanent computer database. These sensor arrays are very thin and very light weight, but they cannot conform to a compound curved surface without wrinkling. Such wrinkled or folded thin film sensor arrays may produce erroneous results. A wrinkled sensor array may produce an electrical signal from the wrinkle alone. Folding or wrinkling also subjects the sensor array to severe fatigue stress, which can lead to early and sudden failure.
A practical sensor array preferably includes a large number of individual force sensors. The problem of electrically connecting this number of individual sensors to data acquisition circuitry is a significant one. Preferably a two-vector array of rows and columns is provided with a top electrode of all sensors in each column commonly connected and a bottom electrode of all the sensors in each row likewise commonly connected.
Insole force sensor arrays are described in U.S. 5,408,873 and U.S. 5,678,448. In addition, alternative apparatuses makes use of a flat pressure sensor array onto which the person places his or her foot as disclosed, for example, in U.S. 5,659,395. Typically the pressure sensor array may be provided by a large number of resistive pressure sensors. This flat plate arrangement has the advantage that there is no bending or folding of the pressure pad which makes the design of the pressure sensors somewhat simpler and allows a high density and a high number of individual pressure sensors to be included within the pressure pad.
Independent of whether an insole pressure device is used or a flat plate, the use of resistive pressure sensors is convenient and provides a light weight apparatus. However, the major disadvantage of resistive pressure sensors is that they are not absolutely calibrated but rather provide relative pressure values. In addition it is possible, through use, that pressure sensors in one area of a pressure path may alter their response characteristic with time. Calibrating or re-calibrating the multitude of pressure sensors in a pressure pad is not only time consuming but also difficult to carry out exactly. As the pressure pad is not absolutely rigid, it is difficult to apply a uniform pressure to individual or groups of sensors.
It is an object to the present invention to provide a pressure distribution sensing apparatus and method for sensing the pressure distribution of a three-dimensional object which avoids or reduces the problems of calibration or re-calibration of pressure known pressure pads.
It is a further object of the present invention to provide a force sensing apparatus which is light in weight and is convenient to use for determining the pressure distribution generated by a three-dimensional object.
The present invention includes a self-calibrating apparatus for measuring the pressure distribution generated by a three-dimensional object, comprising: a plurality of first non-linear force sensors arranged in an array; and a second force sensing device having a linear or linearisable output for measuring the total force applied to the first force sensors.
The present invention includes an apparatus for measuring the pressure distribution generated by a three-dimensional object, comprising: a substantially rigid plate; a plurality of first force sensors arranged in an array, the array of first force sensors being arranged to be on one side of the plate; and a second force sensing device for measuring the total force applied to the plate.
The present invention may also include a method of measuring the pressure distribution generated by a three-dimensional object, comprising the steps of: providing a plurality of first non-linear force sensors arranged in an array; providing a second force sensing device having a linear or linearisable output for measuring the total force applied to the first force sensors; obtaining readings of the force on each sensor of the array of first pressure sensors; adding together the readings from all the first force sensors in the array and comparing this result with the output of the second force sensing device.
The present invention may also include a method of measuring the pressure distribution generated by a three-dimensional object, comprising the steps of: providing a substantially rigid plate; providing a plurality of first force sensors arranged in an array, the array being arranged to be on one side of the plate; providing a second force sensing device for measuring the total force applied to the plate; scanning the array of first force sensors to obtain readings of the force on each sensor; adding together the readings from all the first force sensors in the array and comparing this result with the output of the second force sensing device.
The present invention may provide the advantage that the relative pressure measurements generated by a resistive pressure pad may be continuously and dynamically calibrated by means of a second force sensing device. The second force sensing device is preferably one which is easily calibrated, is linear in its output and is stable. Preferred second sensor devices are an array of strain gauge force sensing devices, piezo-resistive or piezo-electric sensing devices. Preferably the second sensors are read out in parallel. The most preferred second sensing device is one or more piezoelectric sensors. All or any of the apparatus and methods of the present invention may assist in fitting shoes and in gaining physiological data about the human foot or leg. All or any of the apparatus according to the present invention may be provided as a self-contained system.
The dependent claims define individual and separate embodiments of the present invention. The present invention will now be described with respect to the following drawings.